Although many bacteria and protozoa can invade dendritic cells (DC), it is not clear whether infected DC process pathogen-derived antigen and stimulate antigen-specific CD4+ T cells in vivo. To address this issue and obtain data on this question, we have infected susceptible BALB/c (H2-d) mice with a recombinant Leishmania major parasite expressing a fluorescent tracer. We have directly visualized antigen-presenting cells using a mAb reacting to an antigenic peptide derived from the parasite LACK antigen bound to I-Ad MHC class II molecule. I-Ad/LACK complexes were readily detected at the surface of freshly purified parasite-containing DC which expressed a CD8?- CD11b+ F4/80+ gp40- surface phenotype. As demonstrated by electron microscopy some DC contained live amastigotes and expressed I-Ad/LACK complexes in the membrane of the parasitophorous vacuole. Therefore, LACK could be processed and presented to T cells by infected DC in vivo .Furthermore, the presence of I-Ad/LACK complexes on the membrane of parasite-containing phagosomes demonstrated that peptide loading was occuring in this cellular compartment.